The high energy demand associated with open multi-deck refrigerated display cabinets is a direct consequence of their open design. The interaction between the cold refrigerated air inside the cabinet and the relatively warm air of the supermarket takes place across the air curtain, which serves as a non-physical barrier between the customers and the products. It has been estimated that 70% to 80% of the cabinet’s cooling load is due to ambient air infiltration into the cabinet refrigeration apparatus, which was previously entrained through the descending air curtain. A new generation of display cabinets has immerged in recent years, where the display-to-floor area has increased for the sake of maximizing sales. This modification leaves the air curtain with a larger display opening to seal against. Therefore, the design of such cabinets has now become more challenging, especially when attempting to ensure product integrity and temperature homogeneity while attempting to minimize their energy consumption. In this work, advanced numerical and experimental techniques have been integrated to quantify and also minimize the entrainment rate through the air curtain and the infiltration rate into open low-front refrigerated display cabinets. Experimentally, the Particle Image Velocimetry (PIV) technique has been used to map the velocity profile along the air curtain while the Infrared (IR) Thermography technique has been used to map the temperature profile across the cabinet. The Computational Fluid Dynamics (CFD) technique has been used in both case and parametric studies after confirming its validation with experiment. CFD was found to be a valuable tool for the simulation of open low-front refrigerated display cabinets, and the credibility of the results was assured when the boundary conditions were fine-tuned by experimental data. This thesis has demonstrated a systematic procedure where the entrainment rate through the air curtain can be quantified. The effect of various Discharge Air Grille (DAG) parameters was studied, and it was found that the entrainment rate is highly sensitive to the velocity profile and magnitude at the DAG. A velocity profile with a ramp shape having the maximum velocity near the cabinet yielded the minimum entrainment rate, hence the cabinet cooling load was reduced. In addition, two techniques were introduced for the determination of the infiltration rate of the cabinet. The first utilises the tracer-gas method to determine the specific amounts of ambient dry air and water vapour entering the evaporator coil, and the second uses psychrometrics to quantify the infiltration load as well as the other cooling load components by identifying the various heat transfer processes encountered during the operation of the cabinet. The ambient air infiltrated into the cabinet, although corresponds to 31% of the total mass flow rate, was found to be responsible for at least 85% of the total cooling load of the cabinet. This indicates that low-front cabinet suffer more from infiltration. The contribution of this work is by providing a better understanding towards the entrainment and infiltration processes related to open refrigerated display cabinets. The new techniques introduced in this work can help designers to better assess the impact of different design parameters and quantify the amounts of the entrainment and infiltration rates associated with open low-front refrigerated display cabinets.

[pt] Configurações novas e tradicionais de sistemas de refrigeração comerciais foram comparadas considerando sua operação com fluidos refrigerantes alternativos. Primeiramente, desenvolveram-se modelos termodinâmicos para o ciclo transcrítico de dois estágios com refrigerante CO2 (CO2 booster) e para o sistema indireto com CO2 bifásico operando como fluido secundário (pumped CO2). Tais tecnologias foram, em seguida, comparadas com o ciclo de expansão direta (DX) por meio do COP e do consumo anual de energia. Nessa análise, R404A, CO2, e misturas não-azeotrópicas de baixo GWP foram utilizados como fluidos refrigerantes. Em segundo lugar, desenvolveu-se modelo de parâmetros concentrados para simular a operação em regime permanente do sistema de expansão direta com múltiplos compressores e evaporadores. O método multizona foi utilizado na modelagem dos trocadores de calor tubo-e-aleta, com a consideração de diferentes tipos de aletas e superfícies internas para os tubos. Resultados da simulação foram comparados com dados experimentais e, em seguida, calculou-se o impacto ambiental do sistema operando com diferentes refrigerantes de baixo GWP, por meio da metodologia LCCP. / [en] Comparison of new and conventional commercial refrigeration systems, operating with typical and alternative refrigerants, was performed. First, thermodynamic models for the pumped CO2 and the CO2 booster cycles were developed. The COP and the annual energy consumption of these novel designs were compared to those of the traditional direct expansion system in different geographic locations, to take into account year-round climate data. Refrigerant R404A, CO2 and new low-GWP non-azeotropic blends were considered as working uids in this analysis. Second, a component-based lumped parameter model to simulate the steady-state operation of a multicompressor multi-evaporator direct expansion system was developed. The modeling effort considered a multizone approach for the tube-and-fin heat exchangers, as well as addressing enhanced internal surfaces and different fin patterns. Predicted results were compared with experimental data, and a life cycle climate performance (LCCP) analysis was performed to compare the environmental impact of new low-GWP refrigerants.